build: wire desktop-bundle pipeline (CI matrix + per-platform installers)

Stand up the seamless-download path for non-technical buyers:

* .github/workflows/build.yml — matrix CI (mac/win/linux) that builds
  PyInstaller bundles and packages them per platform on tag push,
  attaching the resulting installers to a GitHub Release.
* build/installer.iss — Inno Setup script for the Windows installer
  (per-user install, optional desktop shortcut, runs on finish).
* build/macos/build_dmg.sh — wraps DataTools.app into a .dmg with a
  drag-to-/Applications layout.
* build/appimage/{AppRun,datatools.desktop,build.sh} — AppImage recipe.
* src/__init__.py — single source of truth for __version__; the spec
  reads it (was hardcoded), CI passes it through to all packagers.

Buyer download path now lives in the top-level README. Per-build
README documents the Phase 2 step (signing/notarization) that needs
the owner's Apple Developer + Windows code-signing credentials —
those are intentionally not in CI yet because they require setup
outside this repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/installer.iss

@@ -0,0 +1,49 @@
+; Inno Setup script for DataTools — Windows installer.
+;
+; Compile from the repo root:
+;   iscc /DAppVersion=1.0.0 build\installer.iss
+;
+; CI passes the version via /DAppVersion to keep src/__init__.py the
+; single source of truth. Local manual builds: pass /DAppVersion or
+; let the default kick in.
+
+#ifndef AppVersion
+  #define AppVersion "0.0.0-dev"
+#endif
+
+[Setup]
+AppId={{D4A07001-DA7A-4001-8001-DA7A70013700}}
+AppName=DataTools
+AppVersion={#AppVersion}
+AppVerName=DataTools {#AppVersion}
+AppPublisher=DataTools
+AppPublisherURL=https://datatools.app
+DefaultDirName={autopf}\DataTools
+DefaultGroupName=DataTools
+DisableProgramGroupPage=yes
+OutputDir=..\dist
+OutputBaseFilename=DataTools-{#AppVersion}-win-setup
+Compression=lzma2/max
+SolidCompression=yes
+WizardStyle=modern
+ArchitecturesInstallIn64BitMode=x64
+PrivilegesRequired=lowest
+PrivilegesRequiredOverridesAllowed=dialog
+; Allow per-user install (no UAC prompt) when admin isn't available.
+
+[Languages]
+Name: "english"; MessagesFile: "compiler:Default.isl"
+
+[Tasks]
+Name: "desktopicon"; Description: "Create a &desktop shortcut"; GroupDescription: "Additional shortcuts:"
+
+[Files]
+Source: "..\dist\DataTools\*"; DestDir: "{app}"; Flags: recursesubdirs ignoreversion
+
+[Icons]
+Name: "{group}\DataTools"; Filename: "{app}\DataTools.exe"
+Name: "{group}\Uninstall DataTools"; Filename: "{uninstallexe}"
+Name: "{autodesktop}\DataTools"; Filename: "{app}\DataTools.exe"; Tasks: desktopicon
+
+[Run]
+Filename: "{app}\DataTools.exe"; Description: "Launch DataTools"; Flags: nowait postinstall skipifsilent